This invention relates in general to centrifuges. More specifically it relates to a flexible shaft construction for transmitting rotational energy from a motor to a rotor supported on the upper end of the shaft construction. The present invention is an improvement over U.S. Pat. No. 3,938,354 to Lehman.
In conventional centrifuges a shaft couples a rotor supported on the upper end of the shaft to the armature shaft motor. Containers of liquid samples are carried on the rotor. A problem common to such centrifuges is that speed related vibrations develop in the shaft. In particular, each centrifuge design develops characteristic vibrational modes in the shaft which are associated with so-called critical rotational speeds. In the first mode, the center of mass of rotor deflects laterally from an initial position "geometrically centered" with the vertical axis of rotation of the motor armature and the shaft. The deflection causes the shaft to assume a slight, simple curvature; the center of mass of the rotor follows a circular locus about the vertical axis of rotation. As the rotational speed increases to a second critical speed, the shaft enters a second mode of vibration where the shaft assumes a generally S-shaped curvature. The center of mass of the rotor is again laterally offset from the vertical. At even higher speeds, the shaft vibrates in a third mode having a more complex curvature.
Values for the critical speeds and the amplitude of the vibration depend on factors such as the mass and mass distribution of the rotor and the motor, the height of the rotor and motor centers of mass above a motor mount, and the spring characteristics of the mount. The design of the connecting shaft is nevertheless a key factor affecting the performance of a given centrifuge. For example, relatively stiff or thick, short shafts vibrate more severely than relatively long, thin shafts. Such long, thin shafts flex more readily to accommodate vibrations and thereby provide a "smooth ride" for the samples in the rotor. Long, thin shafts are therefore usually employed in centrifuge applications where vibration must be minimized such as the separation of blood.
It is also important to control a "downward" transmission of vibrational energy from the typically heavy, rapidly rotating rotor which can place a severe load on the motor bearings, cause the centrifuge to "walk" along a floor or bench top, or generate a high noise level. A well-known technique to control these problems is to mount the motor on elastomeric members such as rubber springs. Elastomeric mounts cannot, however, control vibrations generated by a high inertia centrifuge if the shaft is not properly constructed and positioned with respect to the motor and the rotor.
While the vibrational advantages of a long, thin, flexible shaft are well known, they are easily damaged by the mechanical stress of loading or replacing the centrifuge rotor. Also, a severe imbalance during operation can permanently deform or shear the shaft. U.S. Pat. No. 2,827,229 describes one arrangement for protecting the shaft against damage. The shaft is encased with a flexible material that turns on ball bearings held in a flexible race attached to a stationary housing. This arrangement does support the shaft, but it is very expensive to manufacture because a large number of parts must be assembled with a high degree of precision. Also, because the bearings confine the shaft the support structure increases the vibrations, particularly at a critical speed. Another solution, to provide a dampening arrangement, results in an unsatisfactory level of vibration in the rotor.
U.S. Pat. Nos. 3,779,451 and the other aforementioned 3,938,354 both to Lehman and commonly assigned with this application, describe more successful solutions. The '451 patent discloses a shaft surrounded by a rubber sleeve that is in turn enclosed in a rigid tubular member. This construction exhibits low vibrational levels, but is not completely satisfactory in preventing a permanent deformation of the shaft.
The '354 patent also uses a rigid tubular member, but it surrounds the shaft with a clearance that allows the shaft to flex freely. The upper end of the shaft is held in a rotor adapter that has a relatively short, downwardly projecting sleeve that extends into the tubular member. A clearance between the rotor adapter sleeve and the tubular member limits lateral movement of the upper end of the shaft to prevent its permanent deformation. While this construction has proven commercially viable, it is limited in application to relatively moderate rotational speeds. Specifically, at speeds at or above the second critical speed where the shaft is in the S-shaped second mode, it has a tendency to shear, particularly when the centrifuge is also accelerating or decelerating sharply. Since for many conventional centrifuges the second critical speed is typically near 1,000 rpm, and it is frequently desirable to operate even relatively small bench top centrifuges at speeds up to 6,000 rpm the Lehman '354 design severely limits the usefulness or reliability of the centrifuge.
It is therefore a principal object of this invention to provide a flexible shaft construction for a high inertia centrifuge that controls vibrations, protects the shaft against permanent deformation due to a mechanical stress on the rotor, and operates without damage to the shaft at speeds at or above the second or third critical speeds.
Another object is to provide a shaft construction that protects the shaft against permanent deformation or shearing during operation due to a severe imbalance.
A further object is to provide a shaft construction that provides a highly smooth ride, does not cause the centrifuge to "walk" and has a low noise level.
Yet another object is to provide a shaft construction that provides high acceleration and deceleration rates that substantially reduce the time required for a centrifuge operation.
A still further object is to provide a shaft construction that accepts a variety of rotors and minimizes the likelihood of damage to the motor due to a severe imbalance condition.
Another object is to provide a shaft construction that has a comparatively long life and a comparatively low cost of manufacture.